The present invention relates to a process for processing natural gas. More particularly, this invention relates to an integrated process for treating natural gas that is to be converted to liquefied natural gas (LNG) on a ship based system.
A new type of vessel is being developed that may revolutionize offshore production of natural gas. The gas industry is currently planning to build a fleet of ships or barges that can be sailed or towed to the site of offshore gas deposits, extract the gas, pretreatment the natural gas to remove impurities, then freeze it to become LNG and then offload the LNG to tankers for shipping to market. It is hoped that these floating liquefied natural gas (FLNG) ships will be cheaper to use than building onshore pretreatment and liquefaction facilities, speed up the time to bring fields on stream and make it economical to exploit small and remote offshore gas fields. It has been estimated that over ⅙ of global gas reserves are in such fields. There are also security advantages to produce gas offshore in some parts of the world instead of in onshore facilities. In addition, the use of FLNG vessels avoids impacting onshore wildlife habitats and the need to move communities due to the onshore space needed for land based facilities.
In LNG service, the natural gas has to be cleaned before it is sent to a liquefaction unit. Generally, the treated gas CO2 concentration has to be below 50 ppm to avoid freezing of CO2 in the liquefaction process. Water is also removed to avoid hydrate formation. The use of an amine solvent is a well known process and is an accepted technology for land based LNG pretreatment. For offshore FLNG service, however, there are at least two problems associated with use of a solvent process. First, footprint and weight are two important parameters for the ship and platform builder. When an acid gas such as CO2 is present at an increased concentration in a natural gas feed, the amine absorption column diameter and the amine solvent circulation rate that is needed significantly increases, which leads to large footprint and heavy weight. Second, motion at sea often generates flow maldistribution inside amine absorber and regenerator. This flow maldistribution results in low separation efficiency of a solvent process. Hence, due to the motion, the natural gas stream after the amine treatment may not be able to meet the stringent specifications of acid gases such as CO2 required by liquefaction.